In wireless communication systems, interference by radio frequency (RF) transmitters for RF communication systems may become an issue.
The purpose of a transmitter in a digital wireless communication system is to convert a digital low-frequency baseband signal into an RF signal while preserving the modulation, and thereby the baseband information. In a full duplex system, a radio receives and transmits information simultaneously, which may be performed according to frequency duplex division (FDD), i.e. the radio transmits on one frequency and receives on another frequency. This means that, despite the fact that receive (RX) and transmit (TX) signals are spaced in frequency by a duplex distance, distortion products caused by nonlinearities in the transmitter may fall into the receive band and raise the over-all receiver noise figure, and cause non-compliance with a transmit spectrum mask. In order to reduce this effect, external cavity filters may be used in radio base stations (RBS) to filter out unwanted components outside wanted transmission bands. However, intermodulation products caused by multiple transmitted signals, e.g. own TX signal in combination with an interferer signal, may also position unwanted tones in the receive band and thus degrade the receiver signal-to-noise-ratio (SNR). These unwanted tones are not possible to filter out by frequency filtering and they are thus to be handled by the receiver.
In a multi operator FDD RBS site solution, each operator uses a unique paired spectrum and the spectrum parts thus become separated in frequency so as not to interfere with each other's operations. There is no joint scheduling required among the operators, meaning that simultaneous transmissions may occur. However, due to non-linear effects caused by for instance excitation of ferromagnetic materials or other physical properties, e.g. metal oxidation, mechanical disruptions, etc., passive intermodulation (PIM) products may be reflected back to or within the RBS site and fall into one of the operator's receive bands and cause desensitization. As a result, the created passive inter-modulation, PIM, products caused by the multi operator transmissions may result in lowered received uplink SNR for some scenarios. The problem may also arise in the case when the same operator uses two different carrier frequencies, e.g. operating in carrier aggregation setup, in any direction. The problem may also affect or be affected by frequencies used for time division duplex (TDD) upon such relations between frequencies.
A normal operation to handle such problems would be to send a service technician to the RBS site to try to find the cause and replace or mend the causing element. However, such an operation may take a while, and there may be a not fully functioning RBS for a longer or shorter time.
In the case when two FDD downlink bands are separated by the same frequency distance as the duplex distance for one of the FDD receiver frequencies, third order PIM products may become collocated within a receive band without the possibility to be suppressed by filtering as discussed above. As a result, the noise level of the affected receiver will increase and lower the received SNR in the uplink.
An example is illustrated in FIG. 1 where two transmitted signals 100, 102, for example 3GPP frequency band 29 and band 17, from the same RBS give rise to a PIM product 104, in this case due third order intermodulation, i.e. at 2·f1-f2, where f1 is the frequency of transmission in B17 and f2 is the transmission frequency in band 29, falling into the receive band 106 of band 17. As a result, in the band 17 the receiver will lower its sensitivity and face reduced coverage. Continuing this example with reference to FIG. 2, a base station 200 is operating a cell 202. The lowered sensitivity will decrease the possible coverage of the cell to a limited coverage 204. This could potentially lead to dropped connections or limited application coverage as illustrated by UE A 206 in FIG. 2, while for UEs closer to the network node 200, e.g. UE B 208, the communication may still work. This may lead to the operator missing traffic (to be billed) and/or less satisfaction by subscribers.
This may become more and more of a problem the more bands that are available, since with more bands there will be larger risks for co-sited RBSs for different operators, which may create PIM affecting any of the operators' receive bands. For the sake of brevity and easier understanding, the issue has been demonstrated above where intermodulation is caused by two frequencies, but similar effects may arise from further used frequencies, and used frequencies inter-modulating with intermodulation products, and so on. That is, the more frequencies used at a network node site, the more likely is it that an intermodulation product affects a receive band.
It is therefore a desire to detect such situations.